1. Field of the Invention
The present invention relates to a guide thimble plug for coupling a guide thimble and a shock absorption tube to a bottom nozzle of a nuclear fuel assembly.
2. Description of the Related Art
As is well known to those skilled in the art, a nuclear reactor is a device in which a fission chain reaction of fissionable materials is controlled for the purpose of generating heat, producing radioactive isotopes and plutonium, or forming a radiation field.
Generally, in light-water reactor nuclear power plants, enriched uranium (U) is used, in which the proportion of U-235 has been increased by 2-5%. To process enriched uranium into nuclear fuel to be used in nuclear reactors, uranium is formed into a cylindrical pellet having a weight of about 5 g. Several hundreds of pellets are retained in a bundle, and are inserted into a zirconium tube under vacuum conditions. A spring and helium gas are placed into the tube, and a cover is welded and sealed onto the tube, thus completing the fuel rod. A plurality of fuel rods constitutes a nuclear fuel assembly and is burned in a nuclear reactor by nuclear reaction.
FIG. 1 illustrates such a nuclear fuel assembly and elements thereof. Referring to FIG. 1, the nuclear fuel assembly includes a frame body and a plurality of fuel rods 1. The frame body includes a top nozzle 4, a bottom nozzle 5, a plurality of support grids 2, a plurality of guide thimbles 3 and a measurement tube 6. The fuel rods 1 are inserted through the support grids 2 and supported by springs (not shown) and dimples (not shown) which are formed in the support grids 2. In order to assemble the nuclear fuel assembly, lacquer is applied to the surfaces of the fuel rods 1 to prevent the fuel rods 1 from being scratched, and to prevent the springs provided in the support grids 2 from being damaged. Thereafter, the fuel rods 1 are installed in the frame body, and then the top and bottom nozzles 4 and 5 are coupled to the guide thimbles 3, thus completing the assembly of the nuclear fuel assembly. The assembled nuclear fuel assembly is tested for distances between the fuel rods, distortion, dimensions including the length, etc., after the lacquer is removed, thus completing the process of manufacturing the nuclear fuel assembly.
Meanwhile, the guide thimbles 3 provide passages into which control rods (not shown) can be inserted, which are used to operate or stop the nuclear reactor or control the output of the reactor. When it is desired to suddenly stop the nuclear reactor, the control rods free-fall into the guide thimbles 3. Here, to absorb impact generated by free-fall of the control rods, a shock absorption tube is provided in the lower end of each guide thimble 3. As shown in FIG. 2A, the shock absorption tube may be formed by reducing the inner and outer diameters of the lower end of a guide thimble 3′. Alternatively, as shown in FIG. 2B, a separate shock absorption tube 7 having a diameter less than the inner diameter of the guide thimble 3 may be inserted into the guide thimble 3. Recently, to increase lateral resistance and for ease of assembly, a double tube structure like that of FIG. 2B, in which a shock absorption tube is manufactured through a separate process and inserted into the guide thimble, is being used more frequently.
Here, in the case of the double tube structure, the guide thimble 3 and the shock absorption tube 7 are coupled to the bottom nozzle 5 by a guide thimble plug C. In detail, as shown in FIG. 3, the guide thimble 3 and the shock absorption tube 7 are coupled to the guide thimble plug C. The bottom nozzle 5 is fastened to the guide thimble plug C by a screw 9. Thereafter, the shock absorption tube 7 is further reliably fastened to the guide thimble 3 by a welding method or a bulging method using plastic deformation of the guide thimble 3 and the shock absorption tube 7. As such, compared to the case of the guide thimble 3′ which is reduced in diameter in the lower end thereof to form the shock absorption tube, the double tube structure including the guide thimble 3 and the shock absorption tube 7 can be more easily manufactured, and resistance with respect to a lateral load can be increased. Thus, the double tube structure 3, 7 has an advantage over the guide thimble 3′ in preventing the nuclear fuel assembly from being bent. Representative examples of the double tube structure of the guide thimble were disclosed in U.S. Pat. No. 4,655,990 entitled “Fuel Assemblies for Nuclear Reactor,” and U.S. Pat. No. 5,068,083 entitled “Dashpot Construction for a Nuclear Reactor Rod Guide Thimble.”
However, in conventional techniques, when a single guide thimble plug is welded to the double tube structure, a welded portion may be deformed, resulting in the assembly of the shock absorption tube and the guide thimble being very difficult. Therefore, the quality of the product is diminished. In other words, in the case where the shock absorption tube and the guide thimble which form the double tube structure are assembled with the single guide thimble plug by welding, the welded portion may be deformed. As a result, when the guide thimble is assembled with other elements of the nuclear fuel assembly, a large load is applied to the nuclear fuel assembly, thus reducing the productivity, and reducing the quality of the product.
That is, when the upper end of the guide thimble plug is coupled to the shock absorption tube 7, if welding is used as a means for coupling, the straightness of the guide thimble and the shock absorption tube can be compromised, or a welded portion of the shock absorption tube is expanded in diameter so that it becomes very difficult to insert the shock absorption tube into the guide thimble. Furthermore, if the guide thimble plug is coupled to the shock absorption tube by force-fitting, when the guide thimble plug is welded to the guide thimble, the shock absorption tube may be undesirably loosened or removed from the guide thimble plug by welding heat.